Sliding bolts belong to the group of so-called rock bolts. Rock bolts are used in mining, tunnel construction and special civil engineering to stabilize the wall of a gallery, tunnel or embankment. For this purpose, a bore that is conventionally between two and twelve meters long is driven from the gallery or tunnel into the rock. Into this bore a rock bolt of corresponding length is then introduced, the end portion of which is permanently fixed in the bore by means of mortar, special synthetic resin adhesives or mechanical bracing. An anchor plate is usually mounted onto the end of the bolt projecting from the bore and is clamped by means of a nut against the wall of the gallery or tunnel. In this way, loads that are effective in the region of the gallery-or tunnel wall are introduced into deeper layers of rock. In other words, with the aid of such rock bolts rock layers that are more remote from the wall are used to transfer loads in order to minimize the risk of a collapse of the gallery or tunnel.
Conventional rock bolts are capable of transferring a maximum load corresponding to their mechanical design and break if this load (so-called load at break) is exceeded. In order as far as possible to prevent such a total failure of a fitted rock bolt that is triggered for example by rock shifts, so-called sliding bolts have been developed, which, if a predetermined load is exceeded, yield to a defined extent, i.e. may increase their length within specific limits, in order to reduce a stress acting in the rock to a level that may still be transferred by the bolt. Such sliding bolts are designed with a preset sliding path that may be travelled if the predetermined load is exceeded, i.e. as a result of the defined yielding under increased load the total length of the sliding bolt may be lengthened by at most this sliding path. It is desirable if by visually inspecting the sliding bolt it is possible to establish rapidly and unambiguously whether a specific sliding bolt has already yielded to the defined extent, i.e. whether its sliding path has been already partially or completely used up, for this information makes it possible firstly to draw conclusions about the occurrence of rock movements and secondly to be better able to plan the time when a fitted sliding bolt possibly has to be exchanged or supplemented by further rock bolts. If namely the sliding path of the sliding bolt has been completely used up and further rock movements occur, the sliding bolt may fail after its load at break is exceeded.